Sex-specific developmental changes in spinal cord pain pathways following neonatal inflammation

Early-life inflammation can have long lasting impact on pain processing and pain behaviours. For example, we have shown neonatal inflammation can result in changes within spinal neuronal networks and altered flinching of the hind paw following formalin injection three weeks later. This suggests mechanisms for altered pain behaviours lie in first and second order neurons in the pain neuroaxis. Exactly how these changes progress during postnatal development is not known. Accordingly, we investigated neuroinflammatory markers in sensory neurons (dorsal root ganglia; DRGs) and spinal cords of Wistar rats (both sexes) after early life inflammation. Rats were injected with LPS or saline on postnatal days (P) 3 and 5. DRGs and spinal cords (SC) were isolated on P7, 13 and 21, and the expression of six inflammatory mediators were quantified via RT-qPCR. In the DRG, four proinflammatory mediators were elevated in P7 rats exposed to LPS. By P13, only two proinflammatory agents were elevated, whereas at P21 the levels of all six inflammatory mediators were similar between LPS and saline-treated rats. There were no sex-specific differences in the expression profile of any mediator in DRGs. In the spinal cord this expression profile was reversed with no change in inflammatory mediators at P7, elevation of two at P13 and four at P21 in LPS treated rats. Interestingly, these differences were greater in the spinal cords of female rats, indicating sex-specific modulation of neuroinflammation even at these early stages of postnatal development. The increased inflammatory mediator profile in the spinal cords of P21 LPS-treated rats was accompanied by sex-specific modulation of astrocytic (GFAP) activation, with females showing an increase and males a decrease in GFAP following LPS exposure. Together, these data indicate sensory neurons are more susceptible to acute inflammation whereas inflammation in the spinal cord is delayed. The sex-specific modulation of inflammation during critical phases of development may help explain altered pain behaviours in adult males and females.